1. Field of the Invention
The present invention relates to a battery charger and a charging method for charging a battery. The present invention relates, in particular, to a battery charger and a charging method suitable for charging a battery, such as a nickel metal hydride battery, which accumulates a large amount of heat during charging.
2. Description of the Related Art
A charging-type battery which can be repeatedly charged is recently used as a power source for a power tool, for example. A nickel metal hydride battery is widely used as the power tool battery of this type. To charge the battery, a battery charger for rapidly charging the battery with high current is used. That is, while a battery is rapidly charged for about 20 minutes and, a power tool can be continuously used while replacing a currently used battery with another one which has been charged.
The inventor of the present invention studied ways to improve the performance of a power tool by using, as a battery, a nickel metal hydride battery. Although the nickel metal hydride battery can provide higher capacity than a nickel-cadmium battery, a large amount of heat is generated during charging. If the battery temperature becomes high due to the generated heat, the electrodes and separators of cells within the battery deteriorate and battery life is shortened. Due to this, the nickel metal hydride battery cannot be rapidly charged with high current unlike the nickel-cadmium battery.
Furthermore, the nickel metal hydride battery is more sensitive to overcharge than the nickel-cadmium battery and overcharging makes the battery life shorter. For that reason, it is required to avoid overcharging the nickel metal hydride battery. As for equipment which does not require battery replacement, to avoid overcharging the battery, charging current and discharging current are integrated and a battery is charged based on the integrated value. Thus, the battery can be fully charged. In case of charging a battery for equipment such as the above-stated power tool which requires replacing one battery with another, however, it has been difficult to fully charge the battery without overcharging it.